1. Field of the Invention
The invention relates to an improved side looking sonar system.
2. Description of the Prior Art
In side looking sonar systems, a transducer on a carrier vehicle periodically projects pulses of acoustic energy to insonify a target area such as the sea bottom, as the carrier proceeds along a course line. Acoustic energy reflected back from the insonified area, and targets on it, is received by a receiver transducer. The receiver beam associated with the receiver transducer is such that the receiver detects reflected acoustic energy from relatively narrow insonified strips during the course of travel.
The received acoustic energy is processed and displayed on a suitable display apparatus. With each pulse transmission and subsequent reception a scan line is produced on the display to build up a picture of the sea bottom in a manner similar to the scanning of a conventional cathode ray beam in a television picture tube with the presentation being a pattern of highlights and shadows analogous to an optically viewed panorama illuminated by side lighting, with objects outlined in such a way as to permit their identification.
For increased resolutions there has been developed a side looking sonar transducer which is of a curved configuration wherein the transducer segments or elements making up the transducer lie along the arc of a circle whose radius is the design altitude and acoustic energy is focused along a line of focus on the sea bottom. These focused transducers must be used at a precise altitude above the sea bottom since excursions above or below the designed altitude tend to defocus the operation and consequently degrade the display.
A side looking sonar system has been developed which provides for a high resolution operation without the requirement for operating at a single precise altitude. The system, described in U.S. Pat. No. 3,950,723 basically utilizes a multi-element straight line transducer with signal processing channels connected to each segment of the transducer to continuously and electronically vary the focus as a function of time. The effect of this operation is in essence to simulate a transducer with a sharp curvature to focus close in early after a transmitted pulse, then to decrease the curvature with time as the return comes in from longer ranges. The signal processing has the additional capabilities of providing multiple receiver beams so as to increase search rate capabilities.
In matching the straight line transducer array to a curved wave front of an acoustic wave being received, delays or phase shifts are introduced between elements in the signal processing with the greatest phase differential being experienced by the end transducer elements and the design of the transducer array is such that each of the elements is of the same size as the end elements. Since each element output signal is processed, a great deal of signal processing components must be provided. The present invention allows for all range focusing and multiple beam formation, within certain limits, and does so with significantly less signal processing hardware requirements.